5. Material–Coolant Interactions

Basic understanding of material–coolant interactions in subcritical and supercritical water is of great importance for verifying the sustainability of supercritical water cooled reactors (SCWRs); these include such interactions as elution and corrosion of nuclear structural materials, and transportation and deposition processes along the cooling circuit. Although elution and corrosion properties in general light water reactors (at conditions from room temperature up to 300 °C) have been intensively investigated, those above 300 °C are still not well known. Thus, in the work described in this chapter, some structural materials such as type 304 and 316 stainless steels (SUS 304, SUS 316), Alloy 625 (Inconel 625) were used and model elution and corrosion experiments were conducted to obtain fundamental water chemistry data in subcritical and supercritical water (temperature range from 250 to 550 °C at 25 MPa, and in various atmospheres (deaerated and dissolved H₂ or O₂ conditions)).

In Sect. 5.1, an in situ measurement of the elution behavior in subcritical and supercritical water was conducted by employing a new supercritical water loop system combined with an elution detection system. Radioactivated specimens were used, and elution behavior was traced by measuring gamma-rays emitted from the eluted material (Co-60) collected at the outlet of an autoclave. As this technique allows detection of the eluted material with extremely high sensitivity, it is expected to quantify the amount of the elution. Development of the new method was discussed and the elution property at various temperatures and water chemistry conditions was described.

In Sect. 5.2, transportation and deposition processes along a cooling tube in heating and cooling stages were investigated by employing another new supercritical water loop system. The thickness of the oxide layer and its elemental composition, chemical composition of the oxide outer layer, and surface structure were observed by Auger electron spectroscopy, Raman spectroscopy and scanning electron microscopy measurements, respectively. Corrosion behavior of the specimens along the loop was discussed. It was determined that the corrosion behavior is temperature dependent, namely position dependent in supercritical cooled water reactors. The authors of the chapter are Yosuke Katsumura and Yusa Muroya.